1. Field of the Invention
The present invention relates generally to compositions and methods for inhibiting caking or clumping in fertilizers, and more particularly to compositions and methods for inhibiting caking or clumping in urea containing fertilizers by modifying the crystal growth of the urea in the fertilizer over time.
2. Description of Related Art
Urea containing fertilizer compositions have been known and in use for many years. Urea-formaldehyde fertilizers in particular provide a high level of nitrogen availability, which is necessary to maintain growth and color of lawn turf. Examples of useful and effective urea-formaldehyde fertilizers which attain these goals, and processes for their preparation are disclosed in U.S. Pat. Nos. 3,076,700 and 3,231,363, issued to Victor A. Renner. Improvements in urea-formaldehyde fertilizers and processes for their preparation and application are taught in such patents as U.S. Pat. No. 3,223,518 issued to Louis I. Hanson, U.S. Pat. No. 4,025,329 issued to Harvey Maurice Goertz, U.S. Pat. No. 6,039,781 to Harvey M. Goertz et al., and U.S. Pat. No. 5,102,440 issued to William Gallant et al.
While urea-formaldehyde fertilizers in general, as exemplified by those disclosed in the above noted patents, have been found to be extremely useful, it has been found that solid, granulated, urea containing fertilizers such as urea-formaldehyde fertilizers have a tendency to cake or clump over time, such as during storage, causing problems in spreading and/or disseminating the resulting caked or clumped fertilizer products.
It has been found that fertilizers, particularly granulated urea containing fertilizers, which are hygroscopic, develop crystals, and particularly crystal bridges as they age under typical storage conditions. Urea crystal growth and the formation of crystal bridges are major factors in causing undesirable caking and/or clumping of urea containing fertilizers. In an attempt to resolve these problems, anti-caking agents and crystal modifiers have been proposed heretofore. However, such previously suggested agents have not satisfactorily solved the caking/clumping problems in urea containing fertilizers.
For example, known anti-caking agents normally have been applied to the surface of the fertilizer particles, and due to inefficient coating techniques, have resulted in only partially coated granules, providing less than effective reduction in caking of the granules. In addition, over time, such surface applied coatings may break away from the granules, and thus the treatment slowly loses effectiveness.
Accordingly, crystal growth in urea containing fertilizers has been found to be a major factor in causing caking of such fertilizers over time. For example, when grown from relatively pure solutions, urea crystallizes in long needles having length to width ratios (“crystal aspect ratios”) that can exceed 50:1. It is believed that these high crystal aspect ratios contribute significantly to the development of hard caking during typical storage of fertilizers, particularly methyleneurea (MU) fertilizers.
Furthermore, when grown from solutions containing methyleneurea (MU) polymer chains, urea also will crystallize into long needles which contribute to the development of hard caking during storage. For example, when the MU polymer concentration approaches about 45% of the total resin nitrogen distribution, the polymer chains can impair the urea aspect ratio to a degree and, thus, somewhat lower crystal aspect ratios of about 30:1 can be observed which have been found to be sufficiently high to develop substantial fertilizer caking in mixtures containing urea mixed with MU polymers.
It is believed that outward growth of such long crystal needles from the surface of fertilizer particles enables them to bond with outwardly growing crystal needles of other particles causing caking or clumping effects. For example, the ability of urea crystals to achieve the crystal aspect ratios described above causes the urea particles to bond with other particles causing the particles to lock together into concrete-like lumps over time.
It has been suggested that biuret can be mixed with urea, resulting in a molecular level modification of its crystal growth habit. Biuret treated urea crystals have many small aspect ratios. However, in the concentrations required for crystal growth modification to substantially reduce caking of the granules, the biuret would be phytotoxic to many turf grasses, and therefore could not be considered as useful for treatment of urea containing fertilizers.